Arylhydrazines, in particular phenylhydrazines, play an important role as intermediates in the preparation of crop protection agents (triazolones), pharmaceuticals from the series of the pyrazolones, such as Antipyrin or Pyramidon (Beyer/Walter, Lehrbuch der organischen Chemie, page 588, 21.sup.st edition (1988), S. Hirzel Verlag Stuttgart) and pyrazolone dyes.
Hitherto, the most important route for preparing arylhydrazines, in particular phenylhydrazines, used the corresponding arylamines. This type of synthesis is demonstrated below, using the preparation of phenylhydrazine starting from aniline as an example. The synthesis route is summarized schematically in abbreviated form by the following equations (1) to (4): ##STR2##
As can be seen from equation (1), aniline is converted into the corresponding diazonium salt which, according to equation (2), is reduced to the corresponding phenylhydrazinedisulfonate using alkali metal sulfite. The phenylhydrazinedisulfonate is subsequently, according to equation (3), hydrolyzed with a mineral acid where, however, not the phenylhydrazine but the corresponding phenylhydrazine salt is formed from which, as described in equation (4), the phenylhydrazine can be liberated using a base, for example NaOH. This manner of preparing phenylhydrazine is described in Houben-Weyl, Methoden der organischen Chemie, volume 10/2, pp. 180 to 191, in particular p. 181 (1967), fourth edition, Georg Thieme Verlag, Stuttgart.
All the reaction steps described in equations (1) to (4) are carried out in the solvent water. In particular, the hydrolysis described by equation (3), which is carried out using aqueous mineral acid, takes place in the presence of water.
Since the intermediates obtained in accordance with equations (1) to (3) are generally salts, i.e. a diazonium salt in equation (1), a hydrazine disulfonate in equation (2) and a hydrazine salt in equation (3), which are generally considered as being readily water-soluble, it appears neither useful nor sensible to use an additional solvent.
In general, the hydrolysis described in equation (3) requires an appropriate excess of mineral acid, which may be very high for a number of cases. However, a high excess of mineral acid also leads to a high loading of the wastewater produced in the hydrolysis, and to corrosion problems in the reactor vessels used for the hydrolysis. A high proportion of mineral acid in the wastewater furthermore leads to a considerable increase of the costs for wastewater disposal and to increased technical expenditure for regenerating the mineral acid.